Top-level reflection: In the daily hurly-burly of competitive business, it's hard to turn your attention to the big picture, especially those parts that aren't directly related to the daily operations. You need to take time periodically to really think about your business--whether it's providing you and your employees with what you want, whether there aren't some innovative wrinkles you can add, whether you're getting all you can out of your products, your sales staff and your markets. Writing your marketing plan is the best time to do this high-level thinking. Some companies send their top marketing people away to a retreat. Others go to the home of a principal. Some do marketing plan development at a local motel, away from phones and fax machines, so they can devote themselves solely to thinking hard and drawing the most accurate sketches they can of the immediate future of the business.
Ideally, after writing marketing plans for a few years, you can sit back and review a series of them, year after year, and check the progress of your company. Of course, sometimes this is hard to make time for (;there is that annoying real world to deal with);, but it can provide an unparalleled objective view of what you've been doing with your business life over a number of years.